Output identifying attributes, when available

[UnitedMarsupials]

If invoked with --unique-attributes specified, the tool should report the given attributes, when listing differences -- to make it easier to locate the reported differences.

(Actual line-numbers would be good too, but that may not be available from lxml.)

For example, some of our ant-files have hundreds of targets -- with unique names. So I run xmldiff --ratio-mode accurate -p --unique-attributes=name ...

Unfortunately, the only guidance I'm getting in the output is differing target's number: [update-attribute, /project/target[165] ...

Counting up to 165 is rather impractical, is not it? It ought to list the uniquely-identifying attributes instead of -- or in addition to -- the number.